Koi herpesvirus disease is a disease that develops in black carps (Magoi) or colored carps (Nishikigoi) by being infected with Koi herpesvirus (KHV). When the disease is developed, the fishes' motion becomes slower or their appetite decreases. There is no remarkable visible symptom, but color degradation or erosion (sore) of brachia is observed. It is a disease developed from immature fishes to mature fishes, with high mortality rate.
The Koi herpesvirus disease was reported for the first time in May 1998, in Israel. Later, also in Israel, the disease has been reported twice, in autumn of the same year and in the next spring, and about 600 tons of carps, including those for exportation, died. The total damage exceeded 4 million U.S. dollars. After that, many onsets have been reported successively in many countries around the world including Israel, Great Britain, Germany, Netherlands, Belgium, United States, Indonesia, and Taiwan.
In November 2003, the Ministry of Agriculture, Forestry and Fisheries of Japan announced that carps suspected of being infected by Koi herpesvirus disease have been observed in Kasumigaura, Ibaraki prefecture. After which, onsets of Koi herpesvirus have been reported in various regions of the country, including Aomori, Yamanashi, Mie, Okayama and Miyazaki. The Ministry of Agriculture, Forestry and Fisheries is currently making efforts to determine the infection pathway of the disease as well as to prevent dispersion of this disease.
Though some vaccines for fish and shellfish have been developed for bacterial infection, there are almost none for viral or parasitic diseases other than the vaccine against iridovirus in some salmonids and Perciformes (for example, see patent document 1).
Generally, vaccines are used to prevent or treat viral infections. Vaccines include inactivated vaccines (Japanese encephalitis, Weil's disease, etc.), toxoids (tetanus, diphtheria, etc.), attenuated vaccines (BCG, polio, etc.), recombinant vaccines (hepatitis B, etc.). Inactivated vaccines and toxoids, which have detoxified exotoxins, are relatively safe vaccines that induce antibody production. Recombinant vaccines on the other hand do not contain impurities compared to inactivated vaccines, and are believed to be safer vaccines. The only vaccines for fisheries currently authorized in Japan are inactivated vaccines against Vibrio disease, alpha-hemolytic Streptococcus disease, iridovirus and beta-hemolytic Streptococcus. 
Although these vaccines can induce antibody production, cellular immunity is hardly induced, which is a deficit. Moreover, it is commercially necessary to obtain large amount of virus to be the antigens for inactivated vaccines and attenuated vaccines, and it is essential to secure appropriate viral particles. Moreover, in many cases, immune effect obtained with attenuated vaccines is maintained for a long period of time, and so are the side-effects or risks. As for inactivated and recombinant vaccines, the maintenance of antigens is thought to be short in hosts, and adjuvants or the like are therefore needed. All these conventional types of vaccines need refrigeration from manufacture until inoculation to a subject. As such, problems of cost increase and decrease in effect are encountered.
Recently, research and development of vaccines are proceeding creating new types of vaccines. DNA vaccine is a leading new generation vaccine done by administering a plasmid DNA encoding an immunogenic protein to the host. With DNA vaccine, disadvantages of conventional vaccines, as described in the following, have been addressed. These DNA vaccines can induce strongly not only humoral immune response but also cellular immune responses; can endow protective ability against infections; can be purified in large amounts; and because it is stable at room temperature or under high temperature, it does not require refrigeration and have a longer shelf-life. Furthermore, construction or change of DNA vaccines can be readily made by genetic engineering, hence the time necessary to develop it is shortened.
As examples, there have been reports on intramuscular injection of a gene encoding glycoprotein, a constituent protein of Rhabdovirus, stimulating immune response of Oncorhynchus mykiss (for example, see non-patent document 1), and of DNA vaccines (for example, see non-patent document 2); of DNA vaccines against viral hemorrhagic septicemia of Paralichthys olivaceus (for example, see patent document 2); or of DNA vaccines against virus encoding apoptosis-inducing protein of infectious hemapatopoietic necrosis (IHN) virus (for example, see patent document 3); or of DNA vaccines for cultured types using genetic expression system that can induce expression of an immunogenic polypeptide (for example, see patent document 4). However, DNA vaccines for stimulating protective immunity against Koi herpes virus of carps have not yet been reported.
[Patent document 1] Japanese Laid-Open Patent Application No. 9-176043
[Patent document 2] Japanese Laid-Open Patent Application No. 2005-112726
[Patent document 3] Japanese Laid-Open Patent Application No. 2002-125674
[Patent document 4] Japanese Laid-Open Patent Application No. 9-285291
[Non-Patent document 1] P. Boudinot et. al, Virology, (USA), 1998, Vol. 249, p. 297-306
[Non-Patent document 2] McLauchlan et. al, Fish and Shellfish Immunology, England, 2003, Vol. 15, p. 39-50